Copy field names in stored fields context

We have to copy the field names otherwise we either have a handle of a
list that a caller might mutate or we might mutate when they aren't
expecting it, or worse, a handle of a list that is not mutable (and we
end up mutating the list).

Relates #44665

server/src/main/java/org/elasticsearch/search/fetch/StoredFieldsContext.java

@@ -67,7 +67,7 @@ public class StoredFieldsContext implements Writeable {
     public StoredFieldsContext(StreamInput in) throws IOException {
         this.fetchFields = in.readBoolean();
         if (fetchFields) {
-            this.fieldNames = (List<String>) in.readGenericValue();
+            this.fieldNames = new ArrayList<>((List<String>) in.readGenericValue());
         } else {
             this.fieldNames = null;
         }